[Chapel Merge] Default to the c-backend for arm based macs

Branch: refs/heads/main
Revision: ae6c5e0
Author: ronawho
Link: Unavailable
Log Message:

Merge pull request #20155 from ronawho/use-c-backend-arm-mac

Default to the c-backend for arm based macs

[reviewed by @benharsh and @mppf]

We're seeing some ABI issues or something with the llvm backend in
#19218. Until we resolve that issue (hopefully soon) just default to the
C backend.

Even though we expect #19218 to be fixed soon, I want to expand nightly
testing (done in this PR) to include the full test suite, and this
allows us to do so.

Part of Cray/chapel-private#3374
Related to #19218

Modified Files:
M util/chplenv/chpl_llvm.py

M util/cron/test-darwin-m1.bash

Compare: https://github.com/chapel-lang/chapel/compare/3cd225cb704a...ae6c5e0a5efd